One of the most important issues in the control of advanced grid connected equipments such as line interactive power processors, intelligent protection systems, and power quality monitors and analyzers is the synchronization with the power signals, i.e., voltage signal(s) and/or current signal(s). Grid voltage waveforms are sinusoidal and balanced under optimal operating conditions, but they are prone to be distorted and unbalanced as a consequence of the effect of nonlinear loads on the grid and grid faults. On its part, line currents are perfectly sinusoidal and balanced only in the case of ideal grids consisting of perfectly balanced linear loads. Therefore, grid-connected equipments should be properly synchronized with the grid variables to stay actively connected under generic operating conditions.
The phase-locked technology has been used during decades in communications, military and aerospace systems to synchronize a local oscillator with some recognizable external signal. Following these examples, advanced grid-connected equipments have conventionally used a phase-locked loop (PLL) to synchronize its internal control system to the grid voltage or current. In three-phase systems, the PLL based on a synchronous reference frame (SRF-PLL) has become a conventional grid synchronization technique. However, the response of the SRF-PLL is unacceptably deficient when the utility voltage is unbalanced.
WO 2008/055499 discloses a real-time grid monitoring system and method based on a frequency locking loop (FLL).
There is still a need for improving grid monitoring systems.